style: use consistently *_loc for locations

Some members are called foo_location, others are foo_loc.  Stick to
the latter.

* src/gram.h, src/location.h, src/location.c, src/output.c,
* src/parse-gram.y, src/reader.h, src/reader.c, src/reduce.c,
* src/scan-gram.l, src/symlist.h, src/symlist.c, src/symtab.h,
* src/symtab.c:
Use _loc consistently, not _location.

src/scan-gram.l

@@ -161,7 +161,7 @@ eqopt    ({sp}=)?
   int context_state PACIFY_CC (= 0);
 
   /* Location of most recent identifier, when applicable.  */
-  location id_loc PACIFY_CC (= empty_location);
+  location id_loc PACIFY_CC (= empty_loc);
 
   /* Where containing code started, when applicable.  Its initial
      value is relevant only when yylex is invoked in the SC_EPILOGUE